To improve thermal efficiency of gasoline internal combustion engines, lean burn is known to give enhanced thermal efficiency by reducing pumping losses and increasing ratio of specific heats. Generally speaking, lean burn is known to give low fuel consumption and low NOx emissions. There is however a limit at which an engine can be operated with a lean air/fuel mixture because of misfire and combustion instability as a result of a slow burn. Known methods to extend the lean limit include improving ignitability of the mixture by enhancing the fuel preparation, for example using atomised fuel or vaporised fuel, and increasing the flame speed by introducing charge motion and turbulence in the air/fuel mixture. Finally, combustion by auto-ignition, or homogeneous charge compression ignition, has been proposed for operating an engine with very lean or diluted air/fuel mixtures.
When certain conditions are met within a homogeneous charge of lean air/fuel mixture during low load operation, homogeneous charge compression ignition (HCCI) can occur wherein bulk combustion takes place initiated simultaneously from many ignition sites within the charge, resulting in very stable power output, very clean combustion and high fuel conversion efficiency. NOx emission produced in controlled homogeneous charge compression ignition combustion is extremely low in comparison with spark ignition (SI) combustion based on propagating flame front and heterogeneous charge compression ignition combustion based on an attached diffusion flame. In the latter two cases represented by spark ignition engine and diesel engine, respectively, the burned gas temperature is highly heterogeneous within the charge with very high local temperature values creating high NOx emission. By contrast, in controlled homogeneous charge compression ignition combustion where the combustion is rather uniformly distributed throughout the charge from many ignition sites, the burnt gas temperature is substantially homogeneous with much lower local temperature values resulting in very low NOx emission.
Engines operating under controlled homogeneous charge compression ignition combustion have already been successfully demonstrated in two-stroke gasoline engines using a conventional compression ratio. It is believed that the high proportion of burnt gases remaining from the previous cycle, i.e., the residual content, within the two-stroke engine combustion chamber is responsible for providing the hot charge temperature and active fuel radicals necessary to promote homogeneous charge compression ignition in a very lean air/fuel mixture. In four-stroke engines, because the residual content is low, homogeneous charge compression ignition is more difficult to achieve, but can be induced by heating the intake air to a high temperature or by significantly increasing the compression ratio. This effect can also be achieved by retaining a part of the hot exhaust gas, or residuals, by controlling the timing of the intake and exhaust valves.
In all the above cases, the range of engine speeds and loads in which controlled homogeneous charge compression ignition combustion can be achieved is relatively narrow. The fuel used also has a significant effect on the operating range; for example, diesel and methanol fuels have wider auto-ignition ranges than gasoline fuel. A further problem is to achieve ignition at a particular time with maintained combustion stability, while avoiding engine knocking and misfiring.
HCCI has no flame propagation, therefore, instead, the combustion is kinetically controlled. The lack of flame propagation causes the temperature distribution in the combustion chamber in contrary to normal flame propagation to be almost homogeneous, leading to NOx emissions reduction from thousands of ppm to an order of ten ppm. Because of the kinetically controlled combustion, the heat release can be very fast which opens the possibility to generate a theoretically perfect Otto (constant volume combustion). Only very lean or diluted (air or residual gas fraction) fuel/air mixtures can be combusted provided that the compression temperature is high enough.
A problem in connection with homogeneous compression ignition is to control the ignition delay i.e. cylinder temperature in a way that the combustion phasing is correct at varying speed and load conditions of the engine.
One way to realize HCCI combustion is to manipulate the compression temperature and degree of dilution i.e. control the ignition delay and reactivity of the fuel/air mixture through different valve timing events possibly in combination with variable valve timing.
HCCI combustion generated using the methods described in the above SE-applications is dependent on the presence of residual gas fraction, which requires a mode change between conventional SI combustion and HCCI combustion to initiate HCCI engine operation.
As stated above, a general problem is the difficulty in controlling HCCI combustion. When the combustion phasing is correct, the engine efficiency is high i.e. fuel consumption is low.
Too early auto-ignition will cause the engine to knock and the engine efficiency to drop. Knocking combustion, resulting from pressure waves caused by the combustion process, is both harmful for the engine and unpleasant for the driver/passenger of the car because of generated engine noise. Too late auto-ignition will cause the engine cycle to cycle variations to increase. Increased cycle-to-cycle variations can cause the engine to knock and misfire. If a misfire occurs during HCCI operation, the engine will die when the auto-ignition is generated by trapped residual gas fraction of the previous combustion cycle. Following a misfire, the temperature of the trapped gas will be insufficient for achieving auto-ignition in the subsequent cycle. When changing mode between conventional SI engine operation and HCCI engine operation the switch is accomplished within one engine cycle. Directly after the mode change the auto-ignition timing may be advanced, whereby it is progressively retarded over a number of subsequent cycles until the combustion has stabilised. This is illustrated in FIG. 1, where an arrow A indicates retardation of subsequent cycles C1, C2, C3, C4. The extremely early combustion phasing of the first few HCCI cycles, have an impact on engine load. This is apparent from FIG. 2, where the load in Net Mean Effective Pressure (NMEP) is shown for a few engine cycles before, during, and after a mode change. Due to better fuel conversion efficiency, the load in HCCI mode is higher compared to SI mode. A more detailed analysis of this problem can be found in the SAE paper 2003-01-0753.
Hence an object of the invention is to control the ignition timing during auto-ignition, which means allows for monitoring of a current combustion and for correction of a subsequent combustion dependent on the outcome of the monitoring process.